Symmetric and asymmetric routing

Routing & Switching
1

HI Team
Could you please tell difference between symmetric and asymmetric routing if packets received under same vrf between different route policies?

2

Hello Kanu

Putting aside VRFs for a moment, let’s first examine the difference between symmetric and asymmetric routing.

Symmetric routing is when traffic between two points follows the same path in both directions, while asymmetric routing allows different paths for forward and reverse traffic. Generally speaking, both of these routing behaviors can be acceptable in many scenarios. However, asymmetric routing can cause problems in situations such as unicast flooding, unicast reverse path forwarding, and hairpinning when using a firewall. In such cases, care should be taken to ensure proper operation.

Now symmetric and asymmetric routing is the same even in the context of VRFs. Now you are asking about policies in the same VRF. By default, a router will have no explicitly configured VRFs, only the global routing table, and all policies are in that domain. So whether you have a VRF explicitly configured or not, the result will be the same. Remember, in the same VRF, route policies are locally significant. That means that they only affect the VRF in which they are configured. Ensuring consistency across routers sharing the same VRF is important to maintain symmetry.

I hope this has been helpful!

Laz

If you have such routing between different route policies in the same VRF,

3

HI Lagapidis
Thanks for your answer.Might be i m wrong what i have understand given below.
i think packets are not received on source RTR from destination if in single vrf we have configured different polices.
Le say we have vrf A .and route polcies we have 1 & 2 on source RTR .if i m exporting my routes under policy 1 and receving from destination from policy 2.whether is would work. Please confirm?

4

Hello Kanu

I’m not sure that I understood your question completely. Are you asking if routes exported under policy 1 (source) and received via policy 2 (destination) in the same VRF A work correctly? And can asymmetric routing occur when using different route policies for import/export within the same VRF?

If that’s the question, then let’s look at it more completely:

In a single VRF, route policies (e.g., route-map, route-policy, or route-target configurations) can influence how routes are exported (advertised) and imported (received). However, route policies in the same VRF typically apply to how routes are exchanged with other VRFs or routing domains (e.g., between VRF A and the global routing table, or between VRF A and another VRF B).

Could the application of policies in a single VRF result in asymmetric routing? Let’s assume that

  • policy 1 controls routes exported from the source router (e.g., filtering/selecting routes advertised to other devices or VRFs).
  • policy 2 controls routes imported into the destination router (e.g., filtering routes received from other devices or VRFs).

Does It Work? Yes, but with the following considerations:

  • If Policy 1 on the source router exports routes, and Policy 2 on the destination router imports those routes (assuming they match the criteria in Policy 2), communication will work symmetrically.
  • However, if Policy 2 on the destination router is more restrictive (e.g., blocking routes advertised by Policy 1), asymmetric routing or connectivity issues like the following may occur:
    • The source can send traffic to the destination (if it has a route).
    • The destination cannot send return traffic (if it lacks the route due to Policy 2 blocking it).
    • Result: Traffic is asymmetric or blackholed.

So it really depends on the actual policies and how they are implemented. Does that make sense?

I hope this has been helpful!

Laz