creating new redundant link to ISP router

[Merlin-Cisco]

I have a l3 switch (3850) to a ISP router which has built in redundancy (fibre and wifi external connecting) managed by the ISP.

The l3 switch is in a stack of 2 and I want to add an additional link to the router for hardware redundancy.

What config will be need based on the diagram below?

[Giuseppe Larosa]

Hello Merlin,

>> The l3 switch is in a stack of 2 and I want to add an additional link to the router for hardware redundancy.

The additional link should be taken from switch member 2 something like

gi2/0/16

However, to understand if you can add a second link to the ISP router you should provide some more info:

what type of IP subnet is currently used between your C3850 switch and the ISP router?

it is a public network like the one in your network diagram ?

What is the subnet mask?

If it is a public IP subnet with /30 subnet all you can do is to configure a L3 port channel with member links the two physical ports on your stack gi1/0/16 and gi2/0/16.

Be aware of the following:

a) the L3 configuration on gi1/0/16 the current used link should be removed and placed under the interface port-channel.

b) you need to work with the ISP tech support in order to have them to configure it for port channeling (if supported by the router)

If the ISP router does not support port channel, you would need to ask a second public IP subnet /30 to the ISP to use on the second link as a routed indipendent link.

In this case you will have two default routes pointing to the next-hops in current and new subnet.

Note: the ISP can require an additional fee for the new public subnet.

Hope to help

Giuseppe

[Merlin-Cisco]

The 3850 is connected to the ISP router with the below config:

interface GigabitEthernet1/0/16
description QuickLink CPE WAN Router
switchport access vlan 902

ABC (Fibre)
Gig 1/0/16 95 R Quickline vrrp1
ABC (Fibre)
Gig 1/0/16 95 R Quickline bridge0

Device ID: ABC (Fibre)
Entry address(es):
IP address: 151.249.1.1
Platform: QuicklineCC, Capabilities: Router
Interface: GigabitEthernet1/0/16, Port ID (outgoing port): vrrp1
Holdtime : 116 sec

Version :
6.35.4 (stable)

advertisement version: 1
Management address(es):

-------------------------
Device ID: ABC (Fibre)
Entry address(es):
IP address: 151.249.1.2
Platform: QuicklineCC, Capabilities: Router
Interface: GigabitEthernet1/0/16, Port ID (outgoing port): bridge0
Holdtime : 113 sec

With this config are you saying it would be best to setup a port channel for gi1/0/16 and gi2/0/16
but would need to check the router supports port channelling? thanks

[Giuseppe Larosa]

Hello Merlin,

I don't understand the provided show output.

Is this the output of show cdp neighbors or show lldp neighbors ?

Why we see two different neighbors out of port gi1/0/16 the current link ?

IP address: 151.249.1.1

>> Interface: GigabitEthernet1/0/16, Port ID (outgoing port): vrrp1

>> IP address: 151.249.1.2

>> Interface: GigabitEthernet1/0/16, Port ID (outgoing port): bridge0

The first line that mentions VRRP (the standard equivalent of HSRP) makes me think that the ISP routers can be two and not only one. (unless VRRP is configured as a preparation to add a second router)

The second entry tells about a bridge0 interface that makes me think the ISP router is using a feature like bridge domains.

If L3 port-channel configuration is possible on the ISP router (and it is only one device) I would go this way.

Hope to help

Giuseppe