Re: ospf using 2 routers and vrrp/hsrp

carl_townshend · ‎05-30-2013

Hi all

just a quick one, if I have 2 multilayer switches using hsrp or vrrp, and a wan router connects to one of them using a vlan for routing. Which router will ospf using for its route? or will it load balance to both of them as they are equal cost and advertising the same routes to the WAN ?

cheers

Carl

Bilal Nawaz · ‎05-30-2013

Hello Carl, you have two multilayer switches, and a WAN router only connects to one of them...

I wouldn't recommend using any FHRP with a routing protocol unless for specific particular exceptions. If the FHRP is used for other purposes like LAN gateway then its fine. In this kind of scenario, I would implement point-to-point ospf links.

There will be only one way traffic flow can go, OSPF will take this route.

Point is, can only be routed one way here, doesnt matter so much about your FHRP if we are not using this for routing.

When you have equal path routes and connected via both links then you could potentially load balance, OSPF is per session basis if i remember correctly.

Hope this helps

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carl_townshend · ‎05-30-2013

hi there

my question more is, if it is connected to one of the routers, will the router load balance the traffic to each switch over the single link as it will see 2 vlan IP addresses ?

cheers

Carl

Bilal Nawaz · ‎05-30-2013

Oh I see where your question is going to... Well its not true load balancing is it (with my loadbalancing cap on), you are always going to end up traversing through one of the multilayer switches arent you.

But in terms of routing the answer is yes...

Logical and physical representation.

By looking at the logical, both routes will be the same distance - theyre all on the same segment too. Because of this, the router will form OSPF adjacencies with both multilayer switches. Therefor, we'll learn routes via both of them if theyre configured to advertise the same routes.

I've just done a quick lab to test this out and demonstrate...

from the WAN router, at the bottom if I did a ping to something that is on the LAN side of the multilayer switches, the traffic is load balanced in the sense that one session will be targeted at .1 and if did another ping, it directed me to .2 and kept switching between them.

from the WAN router:

I'll ping the 1.1.1.3 address which is a host on VLAN 1 which is shared between the L3 switches. (i.e. on one segment)

=============================================================

WAN#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area

* - candidate default, U - per-user static route, o - ODR

P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/24 is subnetted, 1 subnets

O 1.1.1.0 [110/2] via 5.5.5.2, 00:24:11, FastEthernet0/0

[110/2] via 5.5.5.1, 00:24:11, FastEthernet0/0

5.0.0.0/24 is subnetted, 1 subnets

C 5.5.5.0 is directly connected, FastEthernet0/0

WAN#

=============================================================

WAN#ping 1.1.1.3

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.3, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 47/65/94 ms

WAN#

WAN#traceroute 1.1.1.3

Type escape sequence to abort.

Tracing the route to 1.1.1.3

1 5.5.5.1 63 msec 31 msec 47 msec

2 1.1.1.3 78 msec 78 msec 78 msec

WAN#

=============================================================

So let me ping again and see if it takes the other path...

=============================================================

WAN#ping 1.1.1.3

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.3, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 62/68/78 ms

WAN#

WAN#traceroute 1.1.1.3

Type escape sequence to abort.

Tracing the route to 1.1.1.3

1 5.5.5.2 32 msec 62 msec 31 msec

2 1.1.1.3 94 msec 62 msec 94 msec

WAN#

=============================================================

Sure enough, it does. If we kept doing this we would see the routes alternate between them.

Hope this helps

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Bilal Nawaz · ‎05-30-2013

Oh, forgot to mention, with HSRP or VRRP, the host will respond back to the active member and will not load balance between the multilayer switches.

But anything going towards the LAN side from the WAN, will be.

You could achieve this with GLBP.

Please rate useful posts & remember to mark any solved questions as answered. Thank you.

Joseph W. Doherty · ‎05-30-2013

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If you're using a VLAN for routing, OSPF would normally see each other OSPF router as an adjacent neighbor on the shared subnet and as such neighbors would have the same exit interface (the VLAN) it would see them as equal cost. This also assumes you're not trying to form adjacencies using FHRP IPs (nor is this desirable as FHRP moves between routers but OSPF sends its own LSAs).