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Do I have to connect an ISP line into my router?


I have an ISP link provided as an ethernet connection. Currently the link is going directly into the router. Could I instead put the link into my switch? If this is possible how will bgp be affected and how will the router find the ISP connection?

My switch is a layer 2 switch cisco 3500xl series, and my router is a 3600 series.

Thanks for your help



Re: Do I have to connect an ISP line into my router?


You link seems to be a Layer 3 link and that's why it is terminated on a router and you have your BGP running.

You cannot terminate you ISP link on a 3500 Xl switch as it is a layer 2 switch and doenot have any Layer-3 capability.

HTH,please rate if it does.

-amit singh

Re: Do I have to connect an ISP line into my router?

Hi Dan,

If ur Switch doesn't have routing capabilities, I would say u could do it in one case as follows:-

1- u make sure your ISP assigned specific vlan for u, then u connect the cable from the ISP to specific port on ur switch ( Make Sure the port is in trunking mode and it's allowing the vlan specified from the ISP).

2- u take another port from the Switch to ur router (Make Sure the port is access or trunking mode and allowing the Vlan Specified from the ISP).

3- In this case u should create a subinterface in your router with the same vlan in order to establish ur link.

* The above Soulution would allow u to create subinterfaces in ur router and pass the Vlan traffic through the Switch trunk if u have another links and vice verca.

pls rate the post if it helps.


Community Member

Re: Do I have to connect an ISP line into my router?

Hi Mohamed

Thanks for you input.

If I did do the above how would the vlan help in the router finding its default route out? Also how would bgp find its wayt to the router?

Is the ISP sending the BGP packets to a destination IP? IF that is the case then I am beginning to understand your solution.

Thanks again


Re: Do I have to connect an ISP line into my router?

Hi Dan,

when u apply the a bove then the specified vlan is tagged from your premises till reaches your gateway to the ISP, the subinterface assigned in ur router & the IP address assigned at the ISP under the same vlan is like point-to-point IPs,just for communication.

BGP is exterior gateway protocol, in order bgp function proberly, ur router and the gateway router (ISP) should be reachable for each other, however it has a maximum paths as number of hops to reach its neibour.

So simply u could clarify vlan as follows:-

A VLAN is a switched network that is logically segmented by function, project team, or application, without regard to the physical locations of the users. VLANs have the same attributes as physical LANs, but you can group end stations even if they are not physically located on the same LAN segment. Any switch port can belong to a VLAN, and unicast, broadcast, and multicast packets are forwarded and flooded only to end stations in the VLAN. Each VLAN is considered a logical network, and packets destined for stations that do not belong to the VLAN must be forwarded through a router or bridge.

So as described, if the same VLAN assigned in your premises and in the ISP as well u should consider it as a cable directly connected from ur side to the ISP side with advanced features (Security,broadcasting domain).

Now, when the ISP router being reachable from ur side, then BGP advertise BGP table to the neibour as per it's configured. (u can configure neibour as loopback interface or the ip which assigned under ur vlan interface). then bgp forward the traffic to the destination IP (Neibour) which now should be gateway of last resort, so u dont have to configure default route. the same Setup should be used at the ISP but it might be with some BGP restrictions for spoofing Security & another BGP features that u might not need it.

pls rate if it helps,



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