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

[Cisco Moderador]

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 sep up, however, inner workings and predicting the network reconverge might me a challenge specially in failover scenarios.

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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 

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[Hari Vishnu]

Hello,

I've a quick and simple question. But having hard time to get a proper answer.How we can display "Extended BGP community" in Cisco.

Ta

[Hector Gustavo Serrano Gutierrez]

Hello Hari Vishnu,

Referring to BGP-VPNv4 peerings, the command would be:

• show bgp vpnv4 unicast [all | rd | vrf] A.B.C.D

For instance:

PE-11#show bgp vpnv4 unicast all 192.168.1.0
BGP routing table entry for 1:1:192.168.1.0/24, version 17
Paths: (1 available, best #1, table RED)
 Not advertised to any peer
 Refresh Epoch 1
 Local
 12.12.12.12 (metric 11) (via default) from 12.12.12.12 (12.12.12.12)
 Origin incomplete, metric 332800, localpref 100, valid, internal, best
 Extended Community: RT:1:1 Cost:pre-bestpath:128:332800 0x8800:32768:0
 0x8801:1:76800 0x8802:65282:256000 0x8803:65281:1500
 0x8806:0:3232235777
 mpls labels in/out nolabel/20
 rx pathid: 0, tx pathid: 0x0
PE-11#

Best Regards.

[Hari Vishnu]

Sorry. I should have been more clear.

I would like to know "how to display routes with a particular Route Target set (BGP extended community)

NB: With out creating a community list.

Ta

[Hector Gustavo Serrano Gutierrez]

Hello Hari Vishnu,

The only way seems to be configuring an Extended Community List.

- BGP Support for Named Extended Community Lists

For those who don't know, the Community List works very similar to Access Lists, but its purpose is to match on Extended BGP Communities like Route Target, Site-of-Origin and VPN Distinguisher.

It also can match based on Regular Expressions (when configured as "expanded").

Once the BGP Extended Community List is created, the following command will display the BGP-VPNv4 routes matching on it:

PE-11#show bgp vpnv4 unicast all extcommunity-list ?
  WORD  extcommunity-list name

Best Regards.

[Hari Vishnu]

Hello Hector,

I would like to know how I can display BGP routes that are not imported into any VRF's. Specifically I would like to get routes in no-table. Can you please advice..

Eg:

sh ip bgp vpnv4 all prefix

Will display VPNV4 prefixes imported in all VRF's + the no tables onces. How I can specifically get the required info ?

Ta

[Hector Gustavo Serrano Gutierrez]

Hello Hari Vishnu,

Let's take the following scenario as example:

• Since PE2 does not have the VRF BLUE configured (it does not need it) it doesn't have the Route Targets (RT) to import the routes generated from PE1 vrf BLUE. Therefore, the NLRIs are not even in PE2's BGP VPNv4 table. The BGP VPNv4 table does not even have the routes.

• BGP-RR is acting as its name suggest, as BGP Route Reflector. Since that is the case, all the NLRIs will be in its BGP VPNv4 table. Since it should only act as RR, it typically will not have any customer attached to it and no VRF configured. No Routing tables on this device, so we can say all the BGP VPNv4 entries are not imported into any VRF.

• PE2 can still import the NLRIs in the BGP VPNv4 table even without having the necessary Route Targets using the command "no bgp default route-target filter" under the BGP process. It is used for example in BGP Inter-AS Option B scenario. Once configured, the device will populate the BGP VPNv4 table even with out having the VRFs in its configuration. As far as I know, there is no command that can differentiate which of those NLRIs are going into a VRF or not.

• The command "show bgp vpnv4 unicast vrf NAME rib-failure", can help to let you know if a route received via BGP-VPNv4 is not inserted into the Routing Table due to the same route learned via a different mean with a lower Administrative Distance on that VRF.

I hope this helps.

Best Regards.

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[Hari Vishnu]

Many thanks Hector. I guess we can narrow down the search by using RD. That will give me what I'm looking for.

Also can you please brief the significance of Update Groups in sh ip bgp vpn4 all XX output.

Typical output section  will be..

  Advertised to update-groups:
     10         16         29         64         79

Can you please throw some light on this ?

Ta

[Hector Gustavo Serrano Gutierrez]

Hello Hari Vishnu,

The BGP Dynamic Peer-Groups feature (aka "update-group"), automatically calculates, based on the configuration of the neighbors, as to which neighbors can share updates & hence can fall under a same update-group, rather than depending on manual 'peer-group' configuration.

With the Dynamic Peer-Groups, every neighbor is a part of some update-group (equivalent of a peer-group).

There is no limitation in the number of peers in the same update group.

We can know what peers are part of which update-group or see which update-group a peer is in with the "show ip bgp update-group [update-group index]" command.

Using the above mentioned command, for the update-group index seen in
the "Advertised to update-groups" line, you can verify to what BGP peers the prefix is sent to.

The advantage of grouping the BGP peers is that Cisco IOS internally reduces the amount of CPU and memory utilization as updates are being replicated to all peer group members once, instead of being done individually for each peer.

- BGP Dynamic Update Peer-Groups

- BGP Peer Groups

I hope this information has been useful for you.

Best Regards.

PS: I appreciate your participation in this event. If possible, please post your next question along the other ones. Please follow the link clicking the image below.