topic Asymmetric Routing and TCP Syn Check in General Topics

Asymmetric Routing and TCP Syn Check

Anjush — Sat, 07 Sep 2019 03:45:24 GMT

Hello All,

I have a scenario where I will be having two ISP's (ISP-A and ISP-B) connected to the PA Firewalls via eth1/1 and eth1/2 interfaces. Both these Interfaces will be in the same untrust-zone. ISP-A will be the primary one and ISP-B the backup with some prepends and local preference for incoming and outgoing traffic.

However, ISP-B has confirmed that there will be cases where some external users using ISP-B will always prefer to come to Firewalls via ISP-B.This will cause an asymmetric routing where some of the incoming traffic is via ISP-B and outgoing is via ISP-A.

Since both Interfaces are in the same-zone some users have confirmed that session will match and traffic won't drop and Palo can handle the return traffic.
Has anyone configured similar setup successfully? Are there any gotchas with this kind of setup? If anyone can guide me to a formal Palo Guide would be vaulable too.

Thanks
AS

Re: Asymmetric Routing and TCP Syn Check

JAIDEEP — Sat, 07 Sep 2019 20:36:34 GMT

Hi Anjush,

The PAN-OS implementation, the firewall identifies the flow using 6 tuple key:

>Src & Dst IP address

>Src & Dst ports

>Protocol

>Security Zone

So, As long as both the ISP interface are in same zone and the security policies are configured on the basis of security zone only then you would be good.

Re: Asymmetric Routing and TCP Syn Check

myky — Sat, 07 Sep 2019 21:22:52 GMT

FYI:

https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000Cm0xCAC