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Ask the Expert: How to configure and troubleshoot EIGRP, OSPF and BGP.

Cisco Moderador
Community Manager
Community Manager

This topic is a chance to discuss more about about the most widely deployed Routing Protocols in today’s Enterprise Networks, such as EIGRP, OSPF and BGP. The use of dynamic Routing Protocols is necessary in order to keep the network operational, provide scalability and reduce complexity. Routing Protocols are easy to set up, however, inner workings and predicting the network reconverge might be a challenge specially in failover scenarios.

 

To participate in this event, please use the Join the Discussion : Cisco Ask the Expert button below to ask your questions

Ask questions from Monday, August 8 to August 19th, 2016

Featured Expert

Hector Gustavo Serrano is a Customer Support Engineer in the Global Cisco Technical Assistance Center. He is part of the Routing Protocols team, specialized in troubleshooting Routing issues, additionally he educates about best practices of Routing Protocols deployed on customer’s networks. For the last 6 six years he has worked with Cisco platforms for Enterprise Networks, specially focused on US customers. Hector Gustavo also holds a CCIE in Routing & Switching (49102) and a Bachelor’s degree in Telecommunication engineering.

Hector Gustavo might not be able to answer each question due to the volume expected during this event. Remember that you can continue the conversation on the  Contact Center Community 

Find other  https://supportforums.cisco.com/expert-corner/knowledge-sharing.

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Please be sure to rate the Answers to Questions

7 Accepted Solutions

Accepted Solutions

Hello Sarah

You need redistribution when your router has learned a route from a Routing Protocol, let's say EIGRP, and you want to advertise the same route over a different protocol, like OSPF.

Some guidelines to consider when planning redistribution are:

  • You can only redistribute routes that are installed in the routing table
  • Redistribution is not transitive, meaning if you have static route redistributed into EIGRP, and then EIGRP redistributed into OSPF on the same router, that route won't be present in OSPF because originating protocol is 'static', not EIGRP, so in order to inject the same route into OSPF, you must also redistribute static into OSPF.
  • When redistributing routes into OSPF you must include the subnets keyword
  • When redistributing routes into EIGRP you must include metrics (unless is coming from another EIGRP process)

By default:

  • internal BGP routes are not redistributed
  • OSPF external routes are not redistributed
router eigrp 1
network 10.0.0.0
redistribute ospf 1 metric 10000 10 255 1 1500 route-map from-OSPF

router ospf 1
network 11.0.0.0
redistribute eigrp 1 subnets route-map from-EIGRP

ip access-list ROUTES1
permit 10.0.1.0 0.0.0.255

ip prefix-list ROUTES2 seq 5 permit 11.0.2.0/24

route-map from-OSPF

match ip address ROUTES1

route-map from-EIGRP
match ip address prefix-list ROUTES2


The best way to configure redistribution is to use route-maps to control what routes you want to inject and have a granular control by using ip prefix-lists or access list to match the desired routes.

There are several documents describing redistribution available from Cisco site:

-Route Redistribution Explained

-Understanding Redistribution of OSPF Routes into BGP

-Redistributing Routing Protocols

-Route-Maps for IP Routing Protocol Redistribution Configuration

CCIE 52804

View solution in original post

Hello Sarah,

Redistribution is needed to make communication work when more than one Routing Protocol is running on the network. 

Example of a Redistribution Scenario:

As seen on the diagram:

  • R1 is EIGRP peer with R2.
  • R3 is OSPF peer with R2.

In order R1 can communicate with R3, the Router R2 should do mutual Redistribution between EIGRP and OSPF protocols. This is necessary so R1 and R3 can receive the respective routes using the Routing Protocols they are running.

The configuration for Redistribution on R2 would look similar to:

!
router eigrp 1
 redistribute ospf 1
 default-metric 1000000 1 255 1 1500

!
router ospf 1

 redistribute eigrp 1 subnets
!

Few variances in the configuration exist depending on what exact Routing Protocol the Router is running. However, the configuration is similar as the one shown above.

There are many different scenarios where Redistribution comes in handy in real world deployments. Additionally, Redistribution should be used with caution, specially when it is required to be configured in more than one Router (for instance, in failover scenarios), as Routing Loops might be introduced to the network if doing careless.

Good Cisco documentation can be found via:

Redistributing Routing Protocols
http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8606-redist.html

I hope this information has been useful for you.

Best Regards.

View solution in original post

The use of Route tagging and filters are suggested to avoid sub-optimal paths and/or Routing loops in scenarios with Multiple 'mutual' Redistribution points.

Let's take a quick look at the below example:

  • R3 is receiving the 192.168.1.0/24 as EIGRP External (AD 170).
  • The route is forwarded to R1 and R2 via EIGRP.
  • R1 and R2 are mutual redistributing EIGRP & OSPF, so now the 192.168.1.0/24 exists in the OSPF domain.
  • One of the Routers will prefer the 192.168.1.0/24 via OSPF due to the fact that OSPF AD is 110 while EIGRP External is AD 170 (knowing which Router will have the OSPF entry might be unpredictable as it would be a 'race condition').

# Sub-optimal path example.

R1#show ip route
<Snippet>

D EX  192.168.1.0/24 [170/3072] via 10.1.23.3 (to R3), 00:01:33, GigabitEthernet0/0
R1#


R2#show ip route
<Snippet>

O E2 192.168.1.0/24 [110/20] via 10.1.2.1 (to R1), 00:01:34, GigabitEthernet0/1
R2#

Another possibility is that, for some reason, R1 or R2 calculates the path towards the 192.168.1.0/24 network via one of the Routers doing the redistribution and not via R3.

The outcome would be similar to:

# Routing loop example:

R1#show ip route
<Snippet>

D EX  192.168.1.0/24 [170/28416] via 10.1.23.2 (to R2), 00:01:33, GigabitEthernet0/0
R1#


R2#show ip route
<Snippet>

O E2 192.168.1.0/24 [110/20] via 10.1.2.1 (to R1), 00:01:34, GigabitEthernet0/0
R2#

R1#trace 192.168.1.0
Type escape sequence to abort.
Tracing the route to 192.168.1.0
VRF info: (vrf in name/id, vrf out name/id)
1 10.1.23.2 40 msec 48 msec 44 msec
2 10.1.2.1 32 msec 52 msec 52 msec
3 10.1.23.2 88 msec 88 msec 92 msec
4 10.1.2.1 92 msec 44 msec 120 msec
5 10.1.23.2 176 msec 148 msec 180 msec
6 10.1.2.1 160 msec 200 msec 176 msec
7 10.1.23.2 208 msec 244 msec 240 msec
8 10.1.2.1 236 msec 240 msec 240 msec
9 10.1.23.2 308 msec 160 msec 52 msec
10 10.1.2.1 64 msec 56 msec 56 msec
11 10.1.23.2 68 msec 72 msec 68 msec
12 10.1.2.1 72 msec 72 msec 72 msec
13 10.1.23.2 80 msec 80 msec 84 msec
14 10.1.2.1 112 msec 156 msec 92 msec
15 10.1.23.2 92 msec 92 msec 96 msec
16 10.1.2.1 92 msec 104 msec 96 msec
17 10.1.23.2 104 msec 100 msec 156 msec
18 10.1.2.1 108 msec 124 msec 112 msec
19 10.1.23.2 140 msec 116 msec 120 msec
20 10.1.2.1 120 msec 120 msec 116 msec
21 10.1.23.2 132 msec 132 msec 128 msec
22 10.1.2.1 132 msec 128 msec 128 msec
23 10.1.23.2 140 msec 140 msec 156 msec
24 10.1.2.1 260 msec 152 msec 140 msec
25 10.1.23.2 256 msec 148 msec 152 msec
26 10.1.2.1 176 msec 152 msec 156 msec
27 10.1.23.2 188 msec 196 msec 168 msec
28 10.1.2.1 224 msec 164 msec 292 msec
29 10.1.23.2 348 msec 240 msec 308 msec
30 10.1.2.1 296 msec 212 msec 212 msec
R1#

The solution would be to use Route-Tagging or Filters (i.e. Distribute-lists).


A Router/L3 Switch chooses the Routing decision based on:

  1. Prefix length.
  2. Administrative Distance
  3. Metrics

Lets get into more details:

  1. Prefix length. The longest subnet mask is preferred.

For instance:

  • 192.168.10.0/24 entry will be preferred over
  • 192.168.0.0/16 entry. Due to the prefix length.

  2. Administrative Distance (AD). If the Router learns about a destination from more than one routing protocols, the AD is compared and the preference is given to the route with the lower AD.

For instance, with default AD values, EIGRP (internal) has precedence over OSPF which has precedence over EIGRP (External)

EIGRP (internal)       90
OSPF                      110
EIGRP (external)    170


  3. Metrics. Once both previously mentioned conditions are tied and there are multiple paths to the same destination from a single routing protocol, then the multiple paths would have the same administrative distance and the best path is selected based on the metrics.

The path with the lower metric is elected. If there are multiple pats with equal lowest metrics, those can be elected for ECMP (Equal Cost Multi-Path) which is Load Balancing.

Route Selection in Cisco Routers


I hope this information has been useful for you.

Best Regards.

View solution in original post

Hello

BGP supports now route refresh capability, soft reconfiguration is an option when IOS does not support that route refresh capability, with this you can update inbound route policies without interrupting the adjacency, as an example, after applying a route-map.


In the inbound direction, it will process routes to determine those going to Loc-RIB table, this won't purge Adj-RIB-in table because this table contains the unedited information received from peers, in this way it allows to process again the route policies. Because that, the feature consumes a significant amount of memory, since the Adj-RIB-in must remain in memory.


Now, you mention other tables, so let's recall what every one does:

  • Adj-RIB-in contains NLRIs as they are sent by BGP neighbors after routing policies are applied to save memory, but if you use soft reconfiguration it will contain all NLRIs before applying any routing policy, that's the difference.
  • Loc-RIB contains local NLRIs, and also contains NLRIs from neighbors after processing validity check, next-hop reachability and the best path selection algorithm.
  • Adj-RIB-out contains NLRIs after outbound routing policies are processed.

If you need to explore deeper into this, I think should be included in the CCIE R&S books since it's a topic:

3.7 BGP

 3.7.h Implement and troubleshoot other features                   

    • 3.7.h [i] Multipath
    • 3.7.h [ii] BGP synchronization
    • 3.7.h [iii] Soft reconfiguration, route refresh
CCIE 52804

View solution in original post

The information can indeed be found in the Summary section of Chapter 10 in the book:

"IP Routing on Cisco IOS, IOS XE, and IOS XR: An Essential Guide to Understanding and Implementing IP Routing Protocols"

Moving forward, as per RFC4271:

Adj-RIB-In
The Adj-RIBs-In contains unprocessed routing information that has
been advertised to the local BGP speaker by its peers.

As you mention, the behaviour differs depending on the BGP command:

  • neighbor Neighbor-IP soft-reconfiguration inbound

exists on the configuration or not.

Let's see both scenarios:

1) The command is missing.

There are 2 commands that can help us to see what networks our Router is receiving from the BGP peer.

  1. show ip bgp neighbor Neighbor-IP routes
  2. show ip bgp neighbor Neighbor-IP received-routes

The 1st command will tell you the routes that are received from the specified peer AND are passing the filter inspection that may be configured inbound for that peer in the BGP section (like a Route-Map, Prefix-list, etc.)

Being that said, the command will only show the unfiltered inbound routes.

The 2nd command will not work without the soft-reconfiguration inbound command in place, as the Adj-RIB-in data structure would be missing (It is gone after the processing of creating the NLRIs in the BGP Loc-RIB).

Router2#show ip bgp neighbors 10.1.2.1 received-routes 
% Inbound soft reconfiguration not enabled on 10.1.2.1
Router2#

Furthermore, the command 'clear ip bgp 10.1.2.1 soft in' will trigger a Route-REFRESH Message (compliant with RFC2918) that will make the peer to re-send a BGP UPDATE message destined to us and containing the networks (aka NLRIs) it should be advertising.

BGP_peers_01

2) Using the 'soft-reconfiguration inbound' command.

Once the command is in place, the Adj-RIB-in data structure persists.

It is important to remark that the command 'show ip bgp neighbor Neighbor-IP received-routes'  now shows an output.

For instance:

R2#show ip bgp neighbors 10.1.2.1 received-routes 
BGP table version is 4, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 10.1.2.1 0 0 1 i
* 192.168.1.0 10.1.2.1 0 0 1 i

Total number of prefixes 2
R2#

As the RFC4271 mentions, this is "unprocessed" routing information. These are the NLRIs advertised by the peer, filtered or not on its way to the BGP Loc-RIB.

Keeping the Adj-RIB-in data structure means more RAM will be required of course.

Upon a new execution of the  'clear ip bgp 10.1.2.1 soft in' command (let's say to commit the configuration of a new filter in the BGP section) the Router will now look at the Adj-RIB-in for the routes, there is no need now to request the information to the BGP peer since we keep that unfiltered information on our Adj-RIB-in.

I hope this information has been useful for you.

Best Regards.

View solution in original post

Hi 1001QA.Net

Adj-RIB-In is not "retained" when the 'soft-reconfiguration inbound' is not configured, the content is "deposited" or "moved" to the Loc-RIB.

In order to keep a copy of all the routes advertised by a specific peer (filtered or not) use the 'soft-reconfiguration inbound' command.

For BGP troubleshooting, you can use:

  1. show ip bgp neighbor Neighbor-IP routes
  2. show ip bgp neighbor Neighbor-IP received-routes (only available with 'soft-reconfiguration inbound' configured)
  3. show ip bgp
  4. show ip bgp neighbor Neighbor-IP advertised-routes

The use of those is as follows:

1) It shows the routes that, passing the inbound filters, are received from a specific BGP peer.

2) It shows a full copy of all the routes received from a specific BGP peer.

3) It shows all the routes received from all the BGP peers (after passing the inbound filters) and the ones the Router itself is inserting into BGP.

4) It shows all the routes the Router is advertising via BGP to a specific peer (after filtered outbound).

Best Regards.

View solution in original post

Hello mishaal-thabet, thanks for your participation on this event.

Basically, BGP as a Control Plane protocol, ultimately helps the Router to know where to send the traffic next based on the Destination IP address of the IP Header included in the traffic received. This regardless of the type of IP traffic.

We know VoIP is actually just IP traffic.

A cable or fiber cut with the ISP will eventually bring the BGP adjacency down after 180 seconds as it is the Hold Time BGP uses by default. In some scenarios, BGP will immediately detect a network failure when the link used to reach the neighbor goes down (unfortunately, this doesn't always happen). This will start the process of network convergence so your VoIP traffic can reach its destination again via a Backup path.

Regardless of the type of interface that is used to connect to your ISP, the first step that kicks in network convergence is to detect the network failure.

Bidirectional Forwarding Detection (BFD) can be used by BGP to detect network failures quickly so the network re-convergence process can start faster reducing the total outage time.

Bidirectional Forwarding Detection

BFD needs to be running on both BGP peers.

Of course, I am assuming your network has redundancy.

I hope this helps.

Best Regards.

View solution in original post

37 Replies 37

Sarah Staker
Level 1
Level 1

Hello Hector,

I am trying to understand when do we need to use redistribution and what is the most efficient way to redistribute routes from one routing protocol to another. Do you have any guidelines?

Thank you.

- Sarah

Hello Sarah

You need redistribution when your router has learned a route from a Routing Protocol, let's say EIGRP, and you want to advertise the same route over a different protocol, like OSPF.

Some guidelines to consider when planning redistribution are:

  • You can only redistribute routes that are installed in the routing table
  • Redistribution is not transitive, meaning if you have static route redistributed into EIGRP, and then EIGRP redistributed into OSPF on the same router, that route won't be present in OSPF because originating protocol is 'static', not EIGRP, so in order to inject the same route into OSPF, you must also redistribute static into OSPF.
  • When redistributing routes into OSPF you must include the subnets keyword
  • When redistributing routes into EIGRP you must include metrics (unless is coming from another EIGRP process)

By default:

  • internal BGP routes are not redistributed
  • OSPF external routes are not redistributed
router eigrp 1
network 10.0.0.0
redistribute ospf 1 metric 10000 10 255 1 1500 route-map from-OSPF

router ospf 1
network 11.0.0.0
redistribute eigrp 1 subnets route-map from-EIGRP

ip access-list ROUTES1
permit 10.0.1.0 0.0.0.255

ip prefix-list ROUTES2 seq 5 permit 11.0.2.0/24

route-map from-OSPF

match ip address ROUTES1

route-map from-EIGRP
match ip address prefix-list ROUTES2


The best way to configure redistribution is to use route-maps to control what routes you want to inject and have a granular control by using ip prefix-lists or access list to match the desired routes.

There are several documents describing redistribution available from Cisco site:

-Route Redistribution Explained

-Understanding Redistribution of OSPF Routes into BGP

-Redistributing Routing Protocols

-Route-Maps for IP Routing Protocol Redistribution Configuration

CCIE 52804

Hello Sarah,

Redistribution is needed to make communication work when more than one Routing Protocol is running on the network. 

Example of a Redistribution Scenario:

As seen on the diagram:

  • R1 is EIGRP peer with R2.
  • R3 is OSPF peer with R2.

In order R1 can communicate with R3, the Router R2 should do mutual Redistribution between EIGRP and OSPF protocols. This is necessary so R1 and R3 can receive the respective routes using the Routing Protocols they are running.

The configuration for Redistribution on R2 would look similar to:

!
router eigrp 1
 redistribute ospf 1
 default-metric 1000000 1 255 1 1500

!
router ospf 1

 redistribute eigrp 1 subnets
!

Few variances in the configuration exist depending on what exact Routing Protocol the Router is running. However, the configuration is similar as the one shown above.

There are many different scenarios where Redistribution comes in handy in real world deployments. Additionally, Redistribution should be used with caution, specially when it is required to be configured in more than one Router (for instance, in failover scenarios), as Routing Loops might be introduced to the network if doing careless.

Good Cisco documentation can be found via:

Redistributing Routing Protocols
http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8606-redist.html

I hope this information has been useful for you.

Best Regards.

Hi Hector

Does BGP Soft reconfiguration work with RIB Adj In and RIB Adj Out databases?

The documentation says that:

-software outbound reconfiguration is always there and it is done by reseting BGP RIB version to 0. Then there will be a difference between RIB Adj Out version and the BGP RIB and the route will generate updates for the neighbors that have out of sync records

So for the above there is no need of another data structure

-software inbound reconfiguration - for this the documentations says that the router keeps an unfiltered copy of the routes received from its neighbors, which is used to rerun the filtering in order to see what routes are allowed with the new policy.

My question is this: Is this copy of unfiltered routes the same as RIB ADJ In table? If not why can't this be used instead of having a copy of this. The documentation seems to indicate RBB Adj in as a permament data structure not as a structure that is created when the software reconfiguration inbound feature is configured.

Hi Hector,

 Thanks for the opportunity.

1) When should we consider using EIGRP named mode instead of classic mode?

2) In OSPF we have 2 layer hierarchy that non-zero areas should connect to area 0 backbone directly.

If this rule doesn't exist how would it cause loop?

Regards Vinayak

Hello Vinayaka,

1) EIGRP named mode brings advantages over EIGRP classic mode.

For example:

  • The use of EIGRP Wide Metrics allows EIGRP to calculate an appropriate metric which can differentiate the use of high-speed interfaces, such as 10 Gigabit Ethernet or Ether channels on the network, therefore computing the best path accordingly.

This is possible as Wide Metrics feature supports 64-bit metric, the composite cost metric formula was modified and also, the delay is now measured in picoseconds.

EIGRP Wide Metrics

- Enhanced Interior Gateway Routing Protocol (EIGRP) Wide Metrics White Paper

  • Named mode has been extended to support hierarchical configuration. The advantage is that all the EIGRP related commands can now be found under the EIGRP process section, which facilitates identifying EIGRP related commands.
  • EIGRP Security enhancements with the use of SHA2-256 Algorithm for stronger authentication between EIGRP peers.
  • New products which require the use of EIGRP are developed with Named mode in mind, like the iWAN Cisco's solution.

Additionally, a command that might come in handy is: 'eigrp upgrade-cli', which can be used to upgrade from classic mode to named mode smoothly.

EIGRP Classic to Named Mode Conversion

As a conclusion, on new network deployments, I would consider the use of EIGRP named mode. There is nothing wrong with keep running EIGRP classic in stable network deployments though.


2) A Router that is not part of Area 0 is not considered an ABR (Area Border Router). That means the device will not generate the OSPF LSA (Link State Advertisements) Type 3 - Summary necessary to make inter-area communication possible:

In the above topology, R3 will not generate LSAs type 3 - Summary (it is not an ABR) between Area 1 and Area 2, making R4 unreachable from Area 1 and Area 0.

The solution in such scenario would be the use of OSPF Virtual-links.

Now, R4 should be receiving the appropriate LSAs to know the path to communicate with Area 1 and Area 0.

More information about how to configure Virtual Links can be found via:

- OSPF Virtual Link

You would ask, "well, why not configuring a loopback0 in R3 on Area 0, that should make it an ABR, and should start creating LSA type 3 - Summary packets, right?"

Technically yes, however, the real Area 0, (in our example, between R1 and R2) would not accept the LSAs type 3 generated on Area 2, since a Backbone Area (aka Area 0) should only export and not import Type 3 LSAs, therefore breaking full internetwork communication.

That is also an example of a discontiguous OSPF network design.

As per RFC2328, all the OSPF areas should connect to Area 0.

I hope this information has been useful for you.

Best Regards.

Hi Hector,

Can I redistribute routes between OSPF and BGP, EIGRP and BGP,

EIGRP and IS-IS?

Thanks in advance

Hi vasanth77,

1) Yes, you can redistribute between OSPF and BGP. Some quick observations:

  • When redistributing OSPF into BGP, the OSPF External type routes are not redistributed by default.
  • When redistributing any Routing Protocol into OSPF, you should include the 'subnet' parameter at the end of the 'redistribute' statement.

2) Yes, you can redistribute between EIGRP and BGP. Some quick observations:

  • When redistributing from any other Routing Protocol into EIGRP, you should manually set the parameters bandwidth delay reliability load MTU, so the metric can be calculated for the new EIGRP prefixes. Actually, It is always a best practice to do so.

3) IS-IS is unlikely to be found in Enterprise Networks, it is more frequently used in the core of Service Provider environments.

Yes, you can redistribute between EIGRP and IS-IS. Some quick observations:

  • When redistributing IS-IS into EIGRP, by default, only Level-2 routes are redistributed. However, you can still specify to redistribute Level-1 or Level-1-2 too.
  • When redistributing into IS-IS, a default metric value of 0 is used by default.

It is important to remember the rule of thumb that only routes installed in the Routing Table are redistributed.

- Redistributing Routing Protocols

I hope this information has been useful for you.

Best Regards.

Thanks Hector.

No problem vasanth77.

Thanks for participating.

Best Regards.

Hi Experts,

We have a requirement where in we need to place two Linux servers in active/standby mode and the boxes will be geographically separated and each box will be in a separate subnet.

What we want is to provide a VIP (Virtual IP which needs to be floating between two boxes. The VIP address can be in a separate subnet and there are no real hosts in that subnet ) so that the client which needs to  connect to the server will use the VIP address instead of real IP address. The VIP will always be owned by the active box and upon failover we need to move the VIP to new active.

Problem: Can we move a VIP  from one host to another.? if possible,  How the routing is handled here since the VIP is now moved from one host to other.? Whether we need any routing protocol here to indicate the routers  when the subnet got moved..?

 

Please suggest any other alternate solution(if any).

Hello mpbhargav@gmail.com,

Thanks for participating, regarding your questions:

1. Can we move a VIP  from one host to another.?

I wouldn't be able to tell you if nowadays Operating Systems for Servers have that feature incorporated or not. At least in theory, it should be possible. For that to happen, there should be some kind of 'heartbeat' mechanism to make the Servers aware the peer is up or not.

2. How the routing is handled here since the VIP is now moved from one host to other.?

From the Networking/Routing point, a workaround that I can think is that the Router on the Active Server side can advertise a more specific prefix for the VIP of the Active Server, I am referring to a /32 advertisement. If that Router senses the Server is unreachable (maybe monitoring it with an IP SLA + Track configuration pointing to the Server), the Router will stop advertising the /32. The Router on the Standby Server network, should always be advertising a less specific route for the VIP server's network.

That way, as long as the VIP for the Active Server network is advertised, the rest of the network will prefer the path to the Active Server.

Advertising both VIPs at the same with the same prefix-length time will make the network prefer one Server or the other depending on the metric the remaining Routers are calculating to reach the VIP network.

3. Whether we need any routing protocol here to indicate the routers that subnet when the subnet get moved..?

A workaround would be using the one described above using any Routing Protocol, it depends in the detailed network implementation.

I am not sure if LISP technology may be a better option for the described scenario. I still need to go deeper into it.

I hope this helps.

Best Regards.

Hi Hector,

Thank you so much for your inputs.

Yes, with the pacemaker and corosync (which is a cluster monitoring software that implements heartbeat) we can detect  if the peer is up or not.

Regarding my second question,

How the routing is handled here since the VIP is now moved from one host to other.?

Whether the Server needs to advertise a specific prefix towards the router when the server becomes active(by using routing protocols like RIP or OSPF).? or a router itself can be configured monitor a VIP network.? 

Also can you explain more on the following:

"The Router on the Standby Server network, should always be advertising a less specific route for the VIP server's network."

Whether it means the router has to indicate the other routers this is not the preferred route to the destination(in BGP terms insert extra AS numbers in the ASPATH, since its a backup route).? 

Hello mpbhargav@gmail.com,

Let's take a look at the following diagram:

Whether the Server itself advertises its VIP network using a Routing Protocol (I know for sure there is software that can make Servers run Routing Protocols) or the the Router is who advertises the VIP, the idea is the Active side should advertise a more specific prefix-length for the VIP network as seen above.

That way, the rest of the network should prefer the path towards the Server at left side (configured as Primary) as long as its more specific prefix-length is advertised.

Upon a Server failure, the VIP will stop being advertised, causing the network to failover to the next best path towards the VIP network, which resides on the Standby Server at the right side of the topology.

A Router/L3 Switch bases its Routing decision on:

  1. Prefix length.
  2. Administrative Distance
  3. Metrics

About point #1:

  1. Prefix length. The longest subnet mask is preferred.

For instance:

  • 1.1.1.1/32 entry will be preferred over
  • 1.1.1.1/24 entry, due to the prefix length.

- Route Selection in Cisco Routers

The above approach should help you as a starting point.

Details about the implementation should be carefully reviewed as it depends in the existing network deployment. (Is the WAN the Internet, MPLS/L3 VPN, VPLS, etc?)

Adding an AS number in the AS_PATH should for sure make the route less desirable, however, in certain scenarios, some parts of the network can still elect that route to forward the traffic.

I hope this helps.

Best Regards.

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